Device for the amplification of light-sensitive

ABSTRACT

A device for the amplification of optical signals comprises a light-sensitive medium receiving a modulated signal optic wave and a pump wave, in which the response time of the light-sensitive medium is substantially greater than the mean period for the modulation of the signal wave. The device can be applied to optical transmission in telecommunications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to a device for the amplification oflight-sensitive medium optic signals used to amplify a temporallymodulated signal wave at high frequency. According to the invention, theamplification results from wave-coupling phenomena in light-refractingcrystals. This device has a high-pass type of response which permits theamplification of high-frequency modulated signals. The applicationsrelate to the optical processing of the signals and especially to theregeneration and amplification of signals coming from an optic fiber.

2. Description of the Prior Art

Devices for the amplification of light waves using light-sensitiveelements have already been experimented with.

The French patent filed on Feb. 27th, 1981 under the number 81.03989 andpublished under the number 2 500 937 describes an optic device tomaintain a radiant energy pulse circulating in a waveguide. This deviceuses a light-refracting medium, in which the signal to be maintainedinterferes, and a pumping wave. A transfer of energy takes place fromthe pumping beam towards the signal to be maintained.

The French patent filed on Mar. 13th, 1981 under the number 81.01535 andpublished under the number 2 501 872 also describes an optic device forthe real-time amplification of the radiant energy of a beam. Thisamplifier uses a light-sensitive recording material in which a beam tobe amplified and a reference beam interfere. These beams create agrating of indices strata in the light-sensitive medium. Energy istransferred between the reference beam and the target beam, addingenergy to the target beam. To work in optimum energy transferconditions, this amplifier provides for a displacement of theinterference fringes either through a mechanical displacement of thelight-sensitive recording material or through the phase modulation ofone of the beams.

However, prior art system cannot be used to amplify pulse trains thatoccur at high frequencies, such as the pulse trains transmitted inoptical telecommunications. For the inertia of light-sensitive mediadoes not permit the amplification of these pulse trains.

The device of the invention, by contrast, takes advantage of thisinertia to enable the amplification of pulses occurring at highfrequencies.

SUMMARY OF THE INVENTION

The invention therefore pertains to a device for the amplification oflight-sensitive medium optic signals, the said device comprising:

a medium with with light-induced variations in indices;

a first source emitting a signal optic wave towards the light-sensitivemedium;

A second source emitting a pump optic wave also towards thelight-sensitive medium;

with the signal optic wave and the pumping optic wave interfering in thelight-sensitive medium;

a device wherein the signal optical wave is modulated at high frequencyand has a defined wavelength, the pumping optical wave has a veryslightly different wavelength and the response time of the said mediumat recording and erasure has a value which is appreciably greater thanthe mean modulation period of the signal wave.

BRIEF DESCRIPTION OF THE DRAWINGS

The various objects and features of the invention will emerge moreclearly from the following description given by way of example and madewith reference to the appended figures, of which:

FIG. 1 is a two-wave coupling device in a light-refracting crystalaccording to the prior art;

FIG. 2 is a device for the amplification of a pulse train according tothe invention;

FIGS. 3 to 6 are curves of the functioning of the device of theinvention;

FIG. 7 is an examnple of the device of the invention;

FIG. 8 is a explanatory diagram of the device of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The device of the invention is based on two-wave coupling phenomenaobserved in non-linear materials with light-induced index variationssuch as light-refracting crystals, the principle of which is recalled inFIG. 1. A pump wave (or reference wave) I_(R) and a low-intensity signalwave I_(S) interfere in the volume of the light-refracting crystal 1(BSO, Ga, As, etc). The phenomena of the self-diffraction of the pumpwave I_(R) by the grating 10, light-induced in the volume of the crystal1, lead the amplification of the signal wave I_(S) if there is a phaseshift of π/2 between the index grating and the interference figure. Inlight-refracting crystals, it is now well known that this shift isobtained by either of the following techniques:

recording at zero field E_(o) =0 by the diffusion of photocarriers(BaTiO₃, LiNbO₃, KNbO₃), recording under high applied field, for exampleE_(o) =10 kV.cm⁻¹, and displacement of the interference figure atconstant speed. This displacement of the interference figure is obtainedby a frequency shift of one of the pump waves or signal waves (BSO, BGO,etc.).

According to either of these recording techniques, high gains in thelight-refracting amplifier were obtained in continuous mode for lowincident laser power values (I_(o) <1 W.cm⁻²). For example, foroptimized recording conditions in BaTiO₃ or BSO, the intensity of asignal beam is amplified by a factor of 10³ to 10⁴ after energy transferfrom the pump. These amplifying conditions are obtained for aninteraction length of about 5 to 10 mm. for the crystal and for a ratiobetween the intensities of the pump/signal beams of about:

    β=IR.sub.o /IR.sub.o =10.sup.3

Depending on the materials used, the time constant for the establishmentof the light-induced grating varies from 10 ms (BSO) to 1 second(BaTiO₃) for an incident power density on the crystal of 100 mW.cm⁻² atthe Argon laser wavelength λ=514 nm. Although the response time of theamplifier is relatively long, the object of the present inventiuon usesthe inertia of the crystal to obtain the amplification of ahigh-frequency amplitude-modulated signal wave, namely for a modulationreference meeting the following condition f_(m) >>1/πτ; τ being theresponse time of the crystal to recording and erasing.

The diagram of the device according to the invention is indicated inFIG. 2. The signal beam I_(S) is formed by the pulse train (duration t₁; cycle t₁ +t₂) interferes in the light-refracting crystal with areference wave. Depending on the material used, the spatial shiftbetween the interference figure and the light-induced index variation isobtained either by zero field recording (E_(o) =0) by diffusion ofphotocarriers or by recording in an applied field E_(o) ≠0 and shiftingof the interference figure at constant speed as indicated above.

According to the invention, ω_(o) being the mean wavelength of thesignal beam I_(S), the controlled shift of the interference figure isobtained by a frequency shift δω of the reference wave with respect tothe signal wave: δω≃1/τ, τ being the response time of the crystal (BSO,BGO, GaAs etc.).

The response time of the light-refracting material is controlled by theintensity of the pump beam. Typically, the following values are gotcorresponding to an incident intensity on the pump wave of I_(o) =10mW.cm⁻². To illustrate the invention, it is indicated that the responsetime of the light-refracting crystal at recording and erasure may havethe following values:

τ≈1 second for BaTiO₃,

τ≈some 10 ms for BSO, BGO.

Under these recording conditions, a continuous signal beam of very lowintensity is amplified by a factor 10² -10³ after interaction with thepump wave (length of crystal: l≈5-10 mm). The object of the invention,therefore, is to obtain high gains when the signal beam is modulatedtemporally at high frequency. This new type of amplifier with a verywide pass-band (with a high-pass type or response) therefore offers newprospects for the optic processing of the signal and the regeneration ofthe pulses in an optic transmission line.

The interference of the modulated beam with the reference wave createsan interference figure in the volume of the crystal, the modulation rateof which is variable with time. For the signal I_(S) indicated in FIG.3, the modulation rate m shown in FIG. 4 develops cyclically from thevalue m(t)=m (m being the modulation rate corresponding to thecontinuous signal) to the value m(t)=0 (partial erasure of the gratingby uniform illumination with a pump wave); ##EQU1##

This gives m=0.02 for I_(R) =10⁴ ×I_(S).

The distribution of light intensity in the interference fringes iswritten as follows in these conditions;

    I(x,t)=I.sub.o [1+m(t) cos Kx]

    m(t)=m 0<t<t.sub.1

    m(t)=0 t.sub.1 <t<t.sub.1 +t.sub.2

In view of the inertia of the crystal, the index variation Δn induced atthe stationary state in the light-refracting crystal depends only on themean value of the light intensity received by the crystal, namely:##EQU2##

m=rate of modulation corresponding to the continuous signal (t₂ =0;I_(S))Δn: the maximum value of the index variation that can be inducedin the crystal.

Since the crystal records only the mean value in time of theinterference figure, the intensity of the signal beam can be modulatedat high frequency. The index grating is registered for any modulationfrequency f_(m) such that: ##EQU3##

The crystal therefore behaves like a high-pass filter, the cut-offfrequency of which equals:

    f.sub.c =1/2πτ

namely, for τ=10 ms (BSO crystal), f_(c) ≈20 Hz. Theindex grating istherefore recorded in the crystal for any modulation of the intensity ofthe wave at a frequency f_(m) greater than 20 Hz. In particular, thefast modulations (f_(n) MHZ-GHz) are suitable. The grating thus made inthe light-refracting crystal is amplified by wave coupling-phenomena.(The self-diffraction off the pump wave in the light-induced grating).The index variation after crossing the crystal is equal to:

    Δn(z=l)=Δn(z=0)×exp Γl/2

Γ=gain coefficient of light-refracting crystal. The diffraction capacity7 of a grating of this type with a modulation depth that varies veryquickly as a function of z is written. ##EQU4##

The gain coefficient Γ is intrinsic to the crystal used and its valuedepends on the recording conditions of the grating (no fringes, appliedfield, etc.) Values of the gain coefficient Γ ranging from 3 to 20 percm. have been measured in most of the usual light-refracting crystals.

The amplitude of the signal wave transmitted by the crystal afterinteraction with the pump wave can be put in the form: ##EQU5##

R being the amplitude of the reference wave.

The differential gain on the intensity of the signal transmitted istherefore equal to: ##EQU6##

We therefore get the following expression of the gain of the amplifier.##EQU7##

In view of the values of Γ measured in different crystals, ahigh-frequency modulated low-intensity incident signal is amplifiedaccording to the following conditions:

for a crystal length l=1 cm., a periodic pulse ratio t₂ /t₂ =1 and again coefficient of the light-refracting crystal of Γ=4 cm⁻¹, we obtainan amplifier gain of G_(diff) =15.

For a light-refracting crystal gain coefficient with a value of Γ=8cm⁻¹, we get

    G.sub.diff =110.

It will be noted that the signal is amplified around a component with amean intensity equal to:

    I.sub.M ≃I.sub.S exp Γl

The signal amplifier device for an optic transmission line which is thesubject of the present invention is shown schematically in FIG. 7.

The signal wave that takes the shape of a pulse train, coming from thesingle-mode or multiple-mode fibre 505, interferes in the crystal 1 withthe wave coming from a single-mode semiconductor laser 6 that acts as alocal oscillator. The stability of frequency between the two wavesprovides for the recording of the dynamic hologram in thelight-refracting crystal chosen according to its field of spectralsensitivity, (for example GaAs, a light-sensitive crystal at thewavelengths λ=0.85 μm or λ=1.3 μm). The phenomena in which energy istransferred from the pump wave to the signal wave, as explained earlier,enables the amplification of the pulse trains without limiting thepass-band (a function of amplification with high-pass type frequencyresponse curve). The signal thus amplified can be detected on aphotodiode (with a filtering of the continuous component) or, as shownin the figure, it can be re-injected into a single-mode or multiple-modeoptical fiber.

Since the light beam emitted by the fiber 5 is divergent, there is afocusing device 2 that focuses the beam in the light-refractingcrystal 1. A focusing device 3 also focuses the light beam emitted bythe source 6. The light-refracting crystal 1 and the focusing devices 2and 3 are focused so that the light-refracting crystal is located at acommon focusing point of the beams that come from the fiber 5 and thesource 6.

Thus, the phenomena of interference and amplification for energytransfer are at the maximum.

Another focusing device 4 focuses the amplified beam coming from thelight-refracting crystal 1 and focuses it on an input side of the fiber7.

For example, an amplifier with a light-refracting crystal made ofgallium arsenide may be made by providing for a recording of zero-fieldstrata by the diffusion of photocarriers and by using, for the localoscillator, a semiconductor laser with a wavelength /=1.3 um with anemitting power of 1 mW.

A dynamic hologram can be obtained with:

A diameter φ=100 μm;

A strata pitch Λ≃1 μm;

An interaction length of ρ=10 mm.;

The power density applied to the light-refracting crystal is:

    P=10 W/cm.sup.2.

A light-refracting crystal GaAs, with a response time of τ≃1 microsecondhas been chosen.

The frequency stability between the pump and signal waves has been keptbelow (2πτ)⁻¹ :

    Sf<(2πτ).sup.-1

    giving

    Sf<0.2 MHz

Since the light-refracting crystal (GaAs) has a gain coefficient of Γ≃5cm, we get, for the time t₁ and t₂ which are substantially equal, anamplification gain:

    G.sub.diff =2 exp Γl/2

    giving

    G.sub.diff =25

The amplifier is compatible with the use of signal beam coming from amultiple-mode fiber since each component of the plane wave coming fromthe fiber interferes coherently with the pump wave.

The detection is of the homodyne type, namely the signal detectedresults from the coherent superimposition of the two following waves:

Signal wave=S

Self-diffracted wave=√ηP

The self-diffracted wave acts as a local oscillator wave with its phasestrictly matched at every point with that of the incident signal wave(FIG. 8).

In the above embodiment, it has been assumed that the light-sensitivemedium is a light-refracting crystal. However, the invention can also beapplied in general to a device where the light-sensitive medium is madeof a index variation material with a time constant for the establishmentof the phenomenon such as, for example, materials with thermal indexvariations or materials generating carriers in semiconductors.

Furthermore, it is clear that the numerical examples have been givenonly to illustrate the description and that other alternatives may beconsidered without going beyond the scope of the invention.

What is claimed is:
 1. A device for the amplification of light-sensitivemedium optic signals, said device comprising:a medium with light-inducedvariations in indices; a first source emitting a signal optic wavetowards said light-sensitive medium; a second source emitting a pumpoptic wave directly at said light-sensitive medium; with the signaloptic wave and the pumping optic wave interfering in the light-sensitivemedium; wherein the signal optical wave is modulated at high frequencyand has a defined wavelength, the pumping optical wave has a veryslightly different wavelength and the response time of the said mediumat recording and erasure has a value which is greater than the meanmodulation period of the signal wave.
 2. A device for the amplificationof light signals in a light-sensitive medium according to claim 1,comprising a first focusing device that focuses the signal wavesubstantially at the center of the light-refracting medium, a secondfocusing device that also focuses the pump wave substantially at thecenter of the light-sensitive medium, the said center being at thefocusing point of the two signal and pump waves.
 3. An amplificationdevice according to claim 1, wherein the medium with light-induced indexvariations is a light-refracting crystal.
 4. An amplification deviceaccording to claim 1, the medium with light-induced index variations isa a medium with a thermal index variation.
 5. An amplification deviceaccording to claim 1, the medium with light-induced index variations isa medium which generates carriers in a semiconductor.